1. Field of the Invention
The present invention relates to a laser crystallization apparatus and a crystallization method, and more specifically, to a laser crystallization apparatus and a crystallization method in which positioning is performed very quickly and with a high positional accuracy to irradiate laser light for crystallization.
2. Description of the Related Art
A thin film transistor (TFT) formed on a semiconductor film, for example, a silicon film, provided on a large-area substrate, for example, a glass substrate, is used as, for example, a switching device for switched display in an active matrix type liquid crystal display device.
For the crystallization of a non-single crystal semiconductor thin film such as an amorphous or polycrystal semiconductor thin film used to form the thin film transistor, for example, a laser crystallization technique is used, wherein a short-pulse laser light with high energy is used to melt and crystallize an irradiated area of the non-single crystal semiconductor thin film.
Laser crystallization apparatuses presently serving in production employ a method in which crystallizing laser light with a uniform intensity distribution is irradiated to an amorphous silicon film. However, according to the method, the crystal grain size in a crystallized semiconductor film is as small as 0.5 μm or less, and position of the crystallized grains cannot be controlled. Therefore, a crystal grain boundary is present in a channel region of the TFT, so that there is a limitation in performance of the TFT, for example, uneven characteristics.
There has been a demand for a technique to manufacture a high-quality semiconductor film having large crystal grains, in order to improve the performance of the TFT. As a crystallization method satisfying this demand, among various laser crystallization techniques, an attention is particularly focused on phase modulated excimer laser annealing (PMELA) in which excimer laser light having a light intensity distribution in an inverse peak pattern shape generated by phase modulation is irradiated to the non-single crystal semiconductor thin film, thereby crystallizing the same. The PMELA technique is a method in which excimer laser light having a predetermined light intensity distribution is irradiated to the non-single crystal semiconductor thin film so that an irradiated portion of the semiconductor film is melted and crystallized. The excimer laser light having the predetermined light intensity distribution can be obtained by the phase modulation of incident laser light using a phase modulating element, for example, a phase shifter. The non-single crystal semiconductor thin film is, for example, a thin film of amorphous silicon or polycrystal silicon formed on a glass substrate. In the currently developed PMELA technique, an area sized at about several millimeters square is melted and crystallized by one laser irradiation. Owing to the crystallization of the non-single crystal semiconductor thin film, a crystallized silicon thin film with good quality is formed in which crystal grains are sized at several μm to about 10 μm and relatively uniform in size (e.g., refer to “Amplitude and Phase Modulated Excimer-Laser Melt-Regrowth Method of Silicon Thin-Film—A New Growth Method of 2-D Position Controlled Large-Grains—”, published by Kohki Inoue, Mitsuru Nakata and Masakiyo Matsumura in a thesis journal of Institute of Electronics, Information and Communication Engineers, Vol. J85-C, No. 8, pp. 624-629, 2002). It has been proved that the TFT manufactured in the crystallized silicon thin film formed by this technique has stable electric properties.
In crystallization apparatuses of a conventional method, crystallizing laser light irradiates the semiconductor film in a shape of a long rectangle beam (e.g., 500 μm×300 mm) and with a uniform light intensity distribution. Thus, it is technically impossible to position a place where crystal grains are to be formed, and the crystal grain size in the crystallized semiconductor film is as small as 0.5 μm or less. Therefore, it is not required to absolutely positioning the irradiation position of the crystallizing laser light.
Means for irradiating laser light for crystallization based on an alignment marker provided in a semiconductor layer has been described in Jpn. Pat. Appln. KOKAI Publication No. 2003-197521. However, the laser irradiation means is a laser irradiation method for forming a polycrystalline area but not for forming large-diameter crystallized grains in a predetermined position.
On the other hand, the PMELA crystallization technique is presently under development, wherein crystallizing laser light having an inverse-peak-shaped beam profile is generated by using the phase shifter or a diffracting optical element, and irradiated to the non-single crystal semiconductor film. The PMELA crystallization technique has good characteristics such that efficiency of crystallizing laser light used is high, crystals with large grain sizes can be obtained and the positioning of crystal grains is possible. However, a so-called step-and-repeat irradiation method is employed to crystallize a semiconductor film with a large area. That is, the following is repeated: after one irradiation of the laser light to the non-single crystal semiconductor film, the glass substrate is moved to and stopped at the next irradiation position, and then the laser light is irradiated again. Thus, there is a challenge to further improve throughput so that the PMELA crystallization technique becomes a mass-production technique. The present applicant has been developing a technique to industrialize the PMELA crystallization technique, and is developing a crystallization method with a higher throughput.
There are the following requirements to put the PMELA technique having the excellent characteristics as described above into practical use as an apparatus for producing, e.g., liquid crystal panels: positioning and forming crystal grains with an absolute positional accuracy on a micrometer order to form the main part of the TFT; and irradiating the crystallizing laser light so that the positioning and formation of the crystal grains can be repeatedly reproduced very quickly.
It is one of the object of the present invention to provide a laser crystallization apparatus and a crystallization method with a high throughput capable of forming a semiconductor film having a crystallized area with a large crystal grain size at a predetermined position on a processing substrate, i.e., a substrate to be processed, by irradiating pulse laser light having a predetermined light intensity distribution to the processing substrate to melt and crystallize the semiconductor film.